ST. LOUIS — In February, Missouri Attorney General Josh Hawley joined a Republican lawsuit to overturn the Affordable Care Act, and with it protections for Americans suffering from preexisting medical conditions that previously could be excluded from insurance coverage.

Now, running to unseat Democrat Claire McCaskill in one of the nation’s most competitive U.S. Senate races, Hawley is airing a sympathetic ad using the affliction of his 5-year-old son, diagnosed this year with a rare bone disease.

“A preexisting condition. We know what that’s like,” he says in the ad. “I support forcing insurance companies to cover all preexisting conditions.”

After being a drag on Democratic candidates for several election cycles,President Barack Obama’s2010 health care law is now threatening Republicans who have spent years railing against it and seeking its repeal.

Hawley is one of manyRepublicans in key races scrambling to square their opposition to thehealth care overhaul with voters’ strong support for one of its most popular provisions.

In Ohio and Michigan, candidates for governor are being forced to answer for their anti-Obamacare moves as attorneys general. In other states, Republicans are trying to tailor new responses to voters questioning their intentions. President Trump, who has long pushed for the repeal of Obamacare, has begun defending coverage of preexisting conditions, although like most of the Republicans, he has not been specific about how he would make that happen.

Denny Enloe, an independent voter, wholistened last week to Hawley’s campaignremarks,said he agrees with Hawley on issues like guns, abortion and promoting work over welfare. But he was skeptical of Hawley’s support for coverage of preexisting conditions, noting the federal lawsuit to dismantle the health care law.

The suit argues that the law — which expanded health insurance to millions of low-income Americans but also included a controversial mandate that nearly everyone purchase insurance — is no longer constitutional because Congress last year repealed a tax penalty for those forgoing insurance. Hawley said Monday he does not regret joining the lawsuit and says preexisting conditions can be protected in ways apart from the health care law.

“How are you going to tell me you’re going to fix it when you’re on a lawsuit to invalidate the Affordable Care Act?” said Enloe, who has end-stage chronic obstructive pulmonary disease, and watched Hawley’s Facebook video from his living-room couch in the conservative, rural and working-class town of Union.

Sen. Claire McCaskill (D-Mo.) speaks to students and supporters at the University of Missouri at St. Louis on Sept. 11, 2018. (Jeff Roberson/AP)

Democrats have launched a barrage of ads attacking Republicans on the issue, a reflection of the strong voter sentiment in favor of preexisting condition coverage and the issue’s rise in importance this election year.

Three-quarters of Americans say it is “very important” that protections for people with preexisting conditions remain law, according to the most recent Kaiser Family Foundation poll released in September. A July Kaiser poll found that a candidate’s position on preexisting conditions to be the top health care issue for voters, especially among Democrats, independents and those living in battleground areas. Half of Republicans said it was either the most important factor or a very important factor in their vote.

“Preexisting conditions is the ideal issue for Democrats in red states or purple states who are trying to reach independents,” said Drew Altman, president and chief executive of the Henry J. Kaiser Family Foundation, a nonpartisan nonprofit headquartered in San Francisco. “It’s a third-rail issue that’s incredibly resonant with the public.”

In Ohio, Republican gubernatorial candidate Mike DeWine, who as state attorney general had challenged the constitutionality of the ACA in court, notably has not joined Hawley and the coalition of Republican governors and attorneys general in 20 states on the lawsuit.

DeWine said during a September debate with Democratic candidate Richard Cordray that he’s “always been for the coverage of preexisting illnesses.” Yet he has not signed on with Democrats to save the mandate that preexisting conditions remain covered.

Michigan Attorney General Bill Schuette, a Republican running for governor, is straddling a similar line and has not picked a side in the federal lawsuit, even as the Democratic Governors Association released an ad saying he would deny health coverage to Michiganders with lymphoma, cerebral palsy, asthma and multiple sclerosis. Schuette has dismissed such suggestions as a “scare tactic” but has not said precisely how he would protect such patients.

In August, Senate Republicans up for reelection — Dean Heller in Nevada, Roger Wicker in Mississippi and John Barrasso in Wyoming — introduced a bill to require insurers to continue selling plans to people with preexisting conditions should Republicans prevail in the lawsuit. But health policy analysts have said the legislation does not prevent insurers from excluding specific health conditions from coverage.

Republican Senate candidates in Florida, North Dakota and Montana also have penned op-eds and released statements pledging to protect treatment of preexisting conditions.

Republican strategist Doug Heye said candidates need to spell out to voters how exactly they will do so.

“One of the criticisms Republicans have gotten the last two cycles is only talking about what we’re against, not what we’re for,” said Heye, former spokesman for the Republican National Committee. “This issue is picking up throughout the country, and it behooves Republicans to demonstrate some kind of plan for what they will actually do.”

Policy analysts say it would be financially unrealistic to force insurers to cover preexisting conditions without mandating that everyone, including healthy people, buy insurance. That was the argument Democrats made in approving the mandate.

Nationally, more than a quarter of working-age adults have health conditions that could have left them uninsurable before the ACA, according to Kaiser.

In Missouri — a state where nearly a third of non-elderly residents have health conditions that could have resulted in insurance denial, according to Kaiser — McCaskill has taken to asking voters who attend her town halls to stand if they have a preexisting condition. The first time she did it during a state Democratic fundraising dinner in June, few in the ballroom remained in their seats.

McCaskill, whose campaign is highlighting the stories of voters with preexisting conditions, has attacked Hawley for signing on to the lawsuit.

“Josh Hawley’s lawsuit will take away every protection that Missourians have against overreach by insurance companies, including preexisting conditions,” McCaskill said in a September debate. “He wants the whole thing thrown out, and he knows there is nothing there to back it up.”

President Trump, who won Missouri by more than 18 points in 2016, said in September while stumping for Hawley in Springfield that he and Hawley “will always protect preexisting conditions.” The president, whose administration argued in a court brief filed in June that ACA protections for preexisting conditions should be ruled unconstitutional, did not say how.

As health-care worries continue to percolate through the conservative suburban enclaves surrounding St. Louis, a Democratic challenger in the state’s 2nd Congressional District is reaping the reward.

Cort VanOstran, center, a Democrat running for the 2nd District seat in Missouri, talks to a voter in the St. Louis suburb of Glendale on Sept. 24. (Nick Schnelle/For The Washington Post)

When Cort VanOstran canvassed a leafy neighborhood in upper-middle-class Glendale last week in his increasingly competitive bid to unseat three-term Republican Rep. Ann Wagner, several conservative-leaning independent and Republican voters said they would consider backing a Democrat because of the issue.

“You can’t go into a booth blind and say I’m going all Republican,” said Ken Clarke, a 72-year-old Republican who voted for Trump and Wagner in 2016. His daughter-in-law has multiple sclerosis, and in recent years, he said, he has begun to support the concept of universal health care.

“I wish America would somehow get across-the-board insurance for everyone — like Canada,” Clarke said. “Social medicine has got to be the future.”

“When she made that vote, I felt like it was a vote against me and my family,” VanOstran told Sherry Ingram, a 63-year-old grandmother who retired from her job as a Monsanto accounts manager.

Wagner’s vote, he said, prompted him to run for office. His mother, who died of breast cancer two years ago, had health coverage through a plan sold under the ACA in the final years of her life, VanOstran said.

“I really think that most Republicans don’t want people with preexisting conditions to lose their care,” Ingram said, stepping ontothe porch as her daughter’s goldendoodle peered through the glass door.

Wagner last year co-sponsored a House Republican bill to offer insurance to patients with preexisting conditions. But health analysts say the legislation does not bar insurers from charging higher rates based on health factors.

Ingram, an independent voter who considers herself conservative on economic issues but liberal on social issues, said she has yet to settle on either a House or Senate candidate and is open to the Democratic and Republican nominees in both races.

“It’s very important to make sure preexisting conditions are taken care of fairly,” Ingram said.

Hawley, too, faces questions from voters who typically would be open to siding with a Republican.

“Can you tell me your plan to fix health care?” Peggy Vangorder, who described herself as an independent who often votes Republican,typedon Hawley’s Facebook page after watching his 8-minute speech outlining the stakes of next month’s election.

“This scares people who are sick,” another womanwrote, urging him to clarify by getting “some ads out letting people know you do support preexisting conditions.”

Barbara Chopin, second from right, discusses issues with congressional candidate Cort VanOstran, left, during a meet-and-greet with residents in Wildwood, Mo., on Sept. 26. (Nick Schnelle/For The Washington Post)